1. Development of the Side-by-Side Flexible Twin Bicycle
Bicycling is an efficient means of transportation and one of the easiest ways to exercise with many health benefits including improvement in cardio-vascular fitness and stamina. But bicycling is not free of dangers.
According to Schwab (2012) “A quarter of all fatalities and half of all seriously injured in traffic in the Netherlands are bicyclists, . . . more alarming is that in the last 10 years the number of seriously injured bicyclists is steadily increasing. This increase is for a large part among the elderly, where the types of accidents are so-called single vehicle accidents. The bicyclist is not hit by a car or a bus, he just falls over. One aspect of this falling over can be attributed to the stability of the vehicle, the bicycle.”
As an alternative to overcome this safety concern, in December 2012, I joined two Schwinn® Drifter 26″ bicycles (one man, one woman) rigidly side-by-side in parallel. The resulting side-by-side rigid twin bicycle is similar to what is known as a “Quadracycle”.
The benefits of riding a side-by-side rigid twin bicycle include the vertical stability that reduces the likelihood of falls, dual steering controls that can be shared or alternated between riders, the ability of one rider to pedal or apply brakes while the other rests, and most enjoyable, the social ability to hold a conversation while pedaling at a leisure pace.
However, the side-by-side rigid twin bicycle has also the constraints and limitations of the quadracycle. Quadracycles, (“All about bicycles,” n.d.) “have some stability issues, and it is not usually advised to take corners at superior speeds. The passengers need to shift their weight in order to keep the vehicle on the road.” This stability issue while cornering is related to the quadracycle's inability to lean into turns as regular bicycles do.
Another limitation of quadracycles is that they ride best in a straight line and on even and level surfaces. When turning into a surface of different elevation or inclination, for example when going from the road into an inclined carport ramp, if the approach to the ramp is at an angle, the quadracycle will ride on three tires over the transition. When one tire looses contact with the ground, due to the differences in elevation or inclination, the quadracycle frame to undergo bending and twisting stresses that can compromise its integrity and eventually result in failure due of metal fatigue.
In August 2013, not satisfied with the limitations of the side-by-side rigid twin bicycle, I started design of various mechanisms for joining the two bicycles in a flexible manner to avoid the limitations of the rigid quadracycle and allow each bicycle to lean into turns and to pitch and surge to conform to bumps or hollows and differences in elevation in its path, while retaining the vertical stability of the side-by-side rigid twin bicycle.
During September 2013 I developed various design geometries in a 3D CAD program for connecting two bicycles in parallel with pivoted link bars. I selected four horizontal link bars with spherical rod end bearings to connect the two bicycle frames in such a way as to form a flexible box type assemblage with the objective of minimizing deviations of the bicycle frames from parallel. The spherical rod end bearings are commercially available at various sizes and configurations and are used when a precision articulating joint is required.
The initial 3D CAD design effort also included the use of longitudinal torsion bars, compound torsion bars, or vertical diagonal compression springs mounted on concentric tubes to maintain the flexible side-by-side twin bicycle assemblage upright when at rest and still allow for lean, pitch and surge while riding.
I desired to locate the link bar pivot points as near as possible to each bicycle frame centerline. This was to form, in front view, a rectangle with pivoting corners that would fold into a parallelogram, reasoning that said assemblage would not impose bending stresses on the bicycle frames when it leaned into a turn or when it was twisted around the transverse axis when riding over uneven surfaces. Two of the link bars would be located on the front attached at the top and bottom of the head tube of each bicycle. The other two link bars were located at the rear; one connecting seat tubes near the base of the seats, and the other near the rear wheels hubs. The pivot points of the rear bottom link bar had to be located away from the centerline of the frame to clear the wheel hub and chain mechanism and it was not clear if this deviation from the vertical center plane was possible without introducing bending or twisting stresses on the bicycle frames.
To verify the effect of locating the rear bottom link bar pivots away from the vertical center plane I did graphic simulation of the motions in the 3D CAD program. This simulation consisted of leaning, rotating and moving each CAD model bicycle and then rotating the links on the vertical and horizontal planes centered on one of the pivot points to move the other end of the link as near as possible to its initial pivot point on the other bicycle. This simulation proved to be difficult due to the complexity in motion of the different components in 3D.
I found that, after completing a number of iterations, there was always an error in that at least one end of one of the link bars would not fall exactly on its corresponding pivot point. This meant that on an actual assemblage, when said assemblage was leaned into a turn, each of the four link bars would be either in tension or compression. This would impose bending and twisting stresses on each bicycle frame that could eventually result in metal fatigue failures. This is probably one of the reasons why, to my knowledge, there is no functionally successful side-by-side flexible twin bicycle prior art.
As I continued development of the concept I discovered several other reasons why prior art may have also failed, these reasons are discussed later.
To overcome said twisting and bending problem I initially considered mounting one or more link bars with pivot points supported on compression springs to allow the pivot point to move against the springs and relieve some of the tension or compression force to reduce the frame bending or twisting stresses to acceptable levels. But this represented an undesirable mechanical complexity and the uncertainty that the assemblage would not maintain proper alignment under all lean, pitch and surge motions. The construction of pivot points mounted with compression springs is discussed in the detailed description section related to FIGS. 2g and 2h that describe the horizontal diagonal link bars 420a and 420b. 
I came up with the hypothesis that if the four pivot points of each bicycle frame were located on the same plane, a plane not necessarily on the bicycle frame centerline, independent of the resulting difference between link bar lengths, it might reduce or eliminate the error that at least one link bar end would not fall exactly on its corresponding pivot point. The pivot point plane of each bicycle on said assemblage would look like an inclined “V” shape both in front view and in top view. If said hypothesis was true there would be no bending or twisting stresses imposed on the bicycles frames when said assemblage was leaned into a turn or when it rode over an uneven path.
In October 2013 I built a ⅝-scale wood model employing ¼″ spherical rod end bearings to test said hypothesis and found that, although the motions of the assemblage were very complex, the ⅝-scale wood model could be folded around the longitudinal axes of the pivot points until it collapsed flat and could also be twisted around the transverse axis of the assemblage to the limit afforded by the spherical rod end bearings without appreciable resistance. This test confirmed the hypothesis a coplanar pivot point geometry, that is, all pivot points of each bicycle frame located on an inclined plane, independent of the difference between link bar lengths, avoided the introduction of bending and twisting stresses on the bicycles frames. Details of the ⅝-scale wood model construction and the twisting and folding tests are explained in the detailed description of example embodiments.
From November 2013 to January 2014 I fabricated the components to modify the two rigid bicycles into a side-by-side flexible twin bicycle assemblage following coplanar pivot point geometry. Instead of using four link bars as originally planned I used three by substituting the two front link bars on the head tube with one link bar in the middle of the head tube. The reasoning for this change was that three points in space always define a unique plane; the pivot points on three links would always fall on the same plane independent of errors in fabrication.
I started testing of the first prototype early in February 2014 and immediately encountered two problems that rendered the assemblage unrideable. The first problem was progressive misalignment and excessive scrubbing of the front tires even when attempting just to run on a straight line. I initially attributed this to misalignment of the bicycle frames but, after several trials adjusting the length of the link bars to improve the alignment, I realized that this problem was due to twisting of the bicycle frames. The details of this problem are explained in the detailed description of example embodiments. I replaced the single front link bar on the head tube with two link bars, one installed above the top tube and the second below the bottom tube. These two link bars provided enough rigidity to reduce the twisting of the frames to be essentially imperceptible.
The second problem was related to the use of springs mounted on concentric tubes intended to maintain the flexible assemblage upright when at rest but allow for leaning into turns. I found that these springs interfered with the ability to lean into turns and maintain a constant turn radius. I also found that, after removing the springs, the turning behavior of the assemblage was similar to that of a standard single bicycle. Difficulties with springs intended to maintain the flexible assemblage and riders upright when at rest and while riding in a straight line will be explained later in the detailed description section. Essentially all previous art employs springs for this purpose and this is probably another reason why previous art has not been successful.
I refer to the modified assemblage with the four link bars and without springs as the second prototype. I started testing the second prototype early in March 2014 and found that it satisfied the performance conditions desired. The second prototype of the Side-by-Side Flexible Twin Bicycle maintains the benefits of the original rigid side-by-side twin bicycle while avoiding the constraints of the rigid quadracycle. The flexible attribute refers to the ability of the assemblage to be simultaneously or independently operated by one or more driver riders and, while providing the vertical stability of a four-wheel vehicle, allow for the simultaneous leaning in order to enter, execute and exit from turns in a manner similar to riding a typical single bicycle; allows for pitching around the transverse axis to conform to bumps or hollows in the riding path of each bicycle and allows for the independent vertical surge of each bicycle to conform to differences in elevation in the riding path while maintaining a relative parallel position between each bicycle.
2. Prior Art
According to Pressman (2012), “more patents issue on bicycles than anything else”. Judging from the number of patents in the prior art cited below, there has been an intense interest for over a century to develop a viable side-by-side parallel twin bicycle. Multiple designs of rigid and semi rigid assemblies have been proposed, some with the ability to roll around the longitudinal axis, others with the ability to rotate or pitch around the transverse axis and yet others with the ability to allow for vertical surge of each bicycle.
There is a smaller number that have claimed the ability to combine the movements of roll, pitch and surge in one embodiment.
Notwithstanding the number of designs for side-by-side flexible twin bicycles proposed, the lack of a successful, commercially viable flexible twin bicycle with the ability to combine the movements of roll, pitch and surge in one embodiment hints at a number of shortcomings inherent in those designs that to our knowledge have not been overcome by anyone of the previous designs proposed. A tabulation of some U.S. patents prior art is included in Table 1 on page 46.
The fourth column in table 1 includes comments related to shortcomings of the particular prior art embodiment. Some of these shortcomings are evident upon close examination of the figures and the corresponding description of the operation. For example, the “unintentional rigid assemblage” in Riess (1892) relates to torsion springs located in the middle of otherwise unpivoted link bars and the “unintentional rigid assemblage” in Pomerance (1974) is due to a rigid link bar to coordinate the steering of the two bicycles that is connected directly to the inside tips of the center hub of the front tires (FIG. 8b). The Pomerance (1974) arrangement results in an unsteerable assemblage since when a bicycle is steered the tips of the hub of the front tire follow arc trajectories in opposite directions that that rigid link bar would not allow.
Other shortcomings are not evident and were discovered after several attempts to correct related problems during testing of the first prototype. Identifying the root cause of the problem required some additional testing, close observation and modifications. One example is the longitudinal flexing and twisting of individual bicycle frames due to lateral loads from road-induced deformation of tires. I experienced this problem when testing the first prototype and initially attributed it to misalignment of the bicycles. But after several efforts to get the alignment right failed to resolve the problem I concluded that misalignment was an aggravating factor but not the root cause.
This problem was related to the use of a single link bar on the front of the first prototype that, in combination with the two bicycle frames, did not provide the rigidity anticipated. Bicycle frames are triangulated tubular structures that are extremely strong resisting vertical loads. A sudden lateral force on a single bicycle results in falling to the side and is promptly corrected by the rider by “steering into the fall”. The resulting lateral stress on the frame of that single bicycle is not significant. This is not the case when two bicycles are joined together in parallel.
When two bicycles are joined together in parallel the reaction to lateral forces is not that clear since the system is designed “not to fall” and each frame can induce stresses on the other. It seems that all inventors, I included, unconsciously assumed that the structure would be relatively rigid and exposed to minimal lateral forces. That is not the case as will be explained later in the discussion of the operation of the first prototype.
There are prior art embodiments that employ a single link element at the front, for example Chin et al. (2012), and it is claimed this maintains perfect parallel position when the horizontal (lateral) bending torques from the auxiliary tire would probably bend and deform the single link element at the pivot points. There are embodiments with two link elements, one at the front, the other at the rear, for example Underhaug (2010), Pomerance (1974), Ferrary (1967), that are also claimed to maintain perfect alignment, but the bending torques around the longitudinal axis formed by the pivot points of said link elements combined with the lever arms from the longitudinal axes to the contact points of the tires with the road, will probably tend to bend and twist the individual frames and spread or narrow the track of said assemblies.
Another unobvious shortcoming discovered during prototype testing relates to the turning behavior of embodiments that employ springs or other resilient (“sprung”) components to keep the assemblage and riders in a vertical position while “allowing for leaning into turns and to accommodate for bumps or differences in road elevations.” The behavior of these sprung embodiments is not as I had anticipated. This will be explained later in the section discussing the operation of the first and second prototypes.
In summary, based on the experience gained while developing and testing the first and second prototypes of my side-by-side flexible twin bicycle embodiment, I have concluded that several of the previously proposed prior art embodiments listed in table 1 suffer from a number of limitations, disadvantages or shortcomings that result in not meeting the attributes claimed by the inventors. Some of said limitations, disadvantages and shortcomings are related to:                a. Mechanical complexity that may introduce too much play between components and results in undesirable twisting and misalignment of the embodiment.        b. Mechanical complexity that requires complex fabrication methods and result in relatively heavy assemblies.        c. Link geometry pivot points that when the assemblage is attempted to be leaned and turned into corners would result in a rigid not a flexible structure as claimed.        d. Embodiment assemblage structures that introduce repetitive bending and/or twisting stresses to each bicycle frame and the assemblage components. These stresses can eventually result in permanent deformation and/or metal fatigue and failure of said stressed frames and components.        e. Some of the proposed designs employ springs intended to maintain the assemblies plus riders in an upright position while at rest and while riding on a straight path and simultaneously allowing for flexibility to lean into turns and to accommodate varying road conditions. I found that the sprung assemblies do not necessarily behave as claimed by the inventors under said turning or varied road conditions.        f. Vertically unstable assemblies that, while accelerating, turning or braking, could result in potentially dangerous overturning conditions, contrary to the inherently safe design claimed by the inventors.        
Some of the specific limitations, disadvantages and shortcomings of the relevant prior-art listed above are explained within the detailed description of example embodiments.